Papers misrepresented a client's twin (9)
I believe the answer is:
identical
'twin' is the definition.
(identical twin is a kind of twin)
'papers misrepresented a client's' is the wordplay.
'papers' becomes 'ID' (identifying documents are one's 'papers').
'misrepresented' indicates anagramming the letters.
'a'+'client'='aclient'
'aclient' with letters rearranged gives 'entical'.
'id'+'entical'='IDENTICAL'
(Other definitions for identical that I've seen before include "Indistinguishable" , "Equal" , "Selfsame" , "Not any different" , "Like two peas in a pod" .)